docs: def pdfdump(self, filename=None, **kargs):
          """pdfdump(filename=None, layer_shift=0, rebuild=1)
          Creates a PDF file describing a packet. If filename is not provided a temporary file is created and xpdf is called."""
          canvas = self.canvas_dump(**kargs)
          if filename is None:
              fname = get_temp_file(autoext=".pdf")
              canvas.writePDFfile(fname)
              subprocess.Popen([conf.prog.pdfreader, fname+".pdf"])
          else:
              canvas.writePDFfile(filename)